The present invention relates generally to reverse osmosis membranes and more particularly to a novel composite polyamide reverse osmosis membrane and to a method of producing the same.
It is known that dissolved substances can be separated from their solvents by the use of various types of selective membranes, examples of such membranes including microfiltration membranes, ultrafiltration membranes and reverse osmosis membranes. One use to which reverse osmosis membranes have previously been put is in the desalination of brackish water or seawater to provide large volumes of relatively non-salty water suitable for industrial, agricultural or home use. What is involved in the desalination of brackish water or seawater using reverse osmosis membranes is literally a filtering out of salts and other dissolved ions or molecules from the salty water by forcing the salty water through a reverse osmosis membrane whereby purified water passes through the membrane while salts and other dissolved ions and molecules do not pass through the membrane. Osmotic pressure works against the reverse osmosis process, and the more concentrated the feed water, the greater the osmotic pressure which must be overcome.
A reverse osmosis membrane, in order to be commercially useful in desalinating brackish water or seawater on a large scale, must possess certain properties. One such property is that the membrane have a high salt rejection coefficient. In fact, for the desalinated water to be suitable for many commercial applications, the reverse osmosis membrane should have a salt rejection capability of at least about 97%. Another important property of a reverse osmosis membrane is that the membrane possess a high flux characteristic, i.e., the ability to pass a relatively large amount of water through the membrane at relatively low pressures. Typically, the flux for the membrane should be greater than 10 gallons/ft.sup.2 -day (gfd) at a pressure of 800 psi for seawater and should be greater than 15 gfd at a pressure of 220 psi for brackish water. More preferably, the flux for the membrane is at least about 22 gfd at brackish water conditions. For certain applications, a rejection rate that is less than that which would otherwise be desirable may be acceptable in exchange for higher flux and vice versa.
One common type of reverse osmosis membrane is a composite membrane comprising a porous support and a thin polyamide film formed on the porous support. Typically, the polyamide film is formed by an interfacial polymerization of a polyfunctional amine and a polyfunctional acyl halide.
An example of the aforementioned composite reverse osmosis membrane is disclosed in U.S. Pat. No. 4,277,344, inventor Cadotte, which issued Jul. 7, 1981, and which is incorporated herein by reference. The aforementioned patent describes an aromatic polyamide film which is the interfacial reaction product of an aromatic polyamine having at least two primary amines substituents with an aromatic acyl halide having at least three acyl halide substituents. In the preferred embodiment, a porous polysulfone support is coated with m-phenylenediamine in water. After removal of excess m-phenylenediamine solution from the coated support, the coated support is covered with a solution of trimesoyl chloride dissolved in "FREON" TF solvent (trichlorotrifluoroethane). The contact time for the interfacial reaction is 10 seconds, and the reaction is substantially complete in 1 second. The resulting polysulfone/polyamide composite is then air-dried.
Although the Cadotte membrane described above exhibits good flux and good salt rejection, various approaches have been taken to further improve the flux and salt rejection of composite polyamide reverse osmosis membranes. In addition, other approaches have been taken to improve the resistance of said membranes to chemical degradation and the like. Many of these approaches have involved the use of various types of additives to the solutions used in the interfacial polycondensation reaction.
For example, in U.S. Pat. No. 4,872,984, inventor Tomaschke, which issued Oct. 10, 1989, and which is incorporated herein by reference, there is disclosed an aromatic polyamide membrane formed by (a) coating a microporous support with an aqueous solution comprising (i) an essentially monomeric, aromatic, polyamine reactant having at least two amine functional groups and (ii) an amine salt to form a liquid layer on the microporous support, (b) contacting the liquid layer with an organic solvent solution of an essentially monomeric, aromatic, amine-reactive reactant comprising a polyfunctional acyl halide or mixture thereof, wherein the amine-reactive reactant has, on the average, at least about 2.2 acyl halide groups per reactant molecule, and (c) drying the product of step (b), generally in an oven at about 60.degree. C. to 110.degree. C. for about 1 to 10 minutes, so as to form a water permeable membrane.
The amine salt of Tomaschke is a monofunctional, monomeric (i.e., polymerizable) amine. Preferably, said amine salt is a water soluble salt of a strong acid and a tertiary amine selected from the group consisting of a trialkylamine, such as trimethylamine, triethylamine, tripropylamine; an N-alkylcycloaliphatic amine, such as 1-methylpiperidine; an N,N-dialkylamine, such as N,N-dimethylethylamine and N,N-diethylmethylamine; an N,N-dialkyl ethanolamine, such as N,N-dimethylethanolamine; a bicyclic tertiary amine, such as 3-quinuclidinol; and mixtures thereof, or is a quaternary amine selected from at least one member of the group consisting of a tetraalkylammonium hydroxide, such as tetramethylammonium hydroxide, tetraethylammonium hydroxide and tetrapropylammonium hydroxide; a benzyltrialkylammonium hydroxide, such as benzyltrimethylammonium hydroxide, benzyltriethylammonium hydroxide and benzyltripropylammonium hydroxide; and mixtures thereof.
In U.S. Pat. No. 4,983,291, inventors Chau et al., which issued Jan. 8, 1991, and which is incorporated herein by reference, there is disclosed a membrane which comprises a polymerized reaction product within and/or on a porous support backing material. According to the Chau et al. patent, said membrane may be prepared by contacting a porous support with an aqueous solution of a polyamine which may, if so desired, contain a polar aprotic solvent not reactive with the amines, a polyhydric compound and an acid acceptor. The polyhydric compound, which may include ethylene glycol, propylene glycol, glycerin and other longer carbon atom backbone glycols, may be present in the aqueous solution in an amount ranging from about 0.1 to about 50%. Examples of acid acceptors include sodium hydroxide, potassium hydroxide, sodium carbonate, triethylamine and N,N'-dimethylpiperzine. The surface of the coated support is freed of excess solution and thereafter contacted with an organic solution of a polyacyl halide for a period of time sufficient to form a polymerized reaction product within and/or on the support material. The resulting composite is then treated with a hydroxypolycarboxylic acid, polyaminoalkylene polycarboxylic acid, sulfonic acid, amine salts of acids, amino acid, amino acid salt, polymeric acid and inorganic acid, before drying of the membrane.
In U.S. Pat. No. 5,576,057, inventors Hirose et al., which issued Nov. 19, 1996, and which is incorporated herein by reference, there is disclosed a composite reverse osmosis membrane comprising a polyamide type skin layer on a porous support, said membrane being formed by coating a solution A containing a compound having at least two reactive amino groups on the porous support and, thereafter, contacting a solution B containing a polyfunctional acid halide with the coated layer of solution A, wherein the difference between a solubility parameter of solution A and a solubility parameter of solution B is from 7 to 15 (cal/cm.sup.3).sup.1/2.
Examples of the solvent for solution A set forth in Hirose et al. ('057) are a mixed solvent of water and an alcohol such as ethanol, propanol, butanol, butyl alcohol, 1-pentanol, 2-pentanol, t-amyl alcohol, isoamyl alcohol, isobutyl alcohol, isopropyl alcohol, undecanol, 2-ethylbutanol, 2-ethylhexanol, octanol, cyclohexanol, tetrahydrofurfuryl alcohol, neopentyl glycol, t-butanol, benzyl alcohol, 4-methyl-2-pentanol, 3-methyl-2-butanol, pentyl alcohol, allyl alcohol, ethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol, propanediol, butanediol, pentanediol, hexanediol, glycerol, etc.; and a mixed solvent of water and a nitrogen compound such as nitromethane, formamide, methylformamide, acetonitrile, dimethylformamide, ethylformamide, etc. As an example of the mixing ratio of water and the other solvent of solution A, Hirose et al. ('057) states that the mixing ratio of water/ethanol can be selected in the range of (50 to 90)/(50 to 10), and preferably (60 to 90)/(40 to 10). Hirose et al. ('057) elsewhere states that the amine salts described in JP-A-2-187135 (corresponding to U.S. Pat. No. 4,872,984), such as a salt of a tetraalkylammonium halide or a trialkylamine and an organic acid can also be suitably used for the solution A to facilitate the film formation, improve the absorption of the amine solution in the support, and accelerate the condensation reaction.
In U.S. Pat. No. 5,614,099, inventors Hirose et al., which issued Mar. 25, 1997, and which is incorporated herein by reference, there is disclosed a composite reverse osmosis membrane having a polyamide type skin layer whose average surface roughness is at least 55 nm. The polyamide type skin layer comprises the reaction product of a compound having amino groups and a polyfunctional acid halide compound having acid halide groups. A polymer film may be formed by contacting a solution containing, for example, m-phenylenediamine with a porous polysulfone supporting film so as to form a layer of the solution on the supporting film, then contacting the film with a solution of trimesic acid chloride and holding the film in a hot air dryer so that a polymer film is formed on the supporting film. The surface of the polyamide type skin layer can also be treated with quaternary ammonium salt and coated with a cross-linked layer of an organic polymer having positively-charged groups.
According to Hirose et al. ('099), the compound having multiple amino groups is preferably present in a solution A, said solution A comprising a compound having a solubility parameter of 8-14 (cal/cm.sup.3).sup.1/2, said compound being selected from the group consisting of certain alcohols, ethers, ketones, esters, halogenated hydrocarbons and sulfur-containing compounds. Specific examples of said compound are disclosed in the Hirose et al. ('099) patent. Elsewhere, Hirose et al. ('099) states that the amine salts described in JP-A-2-187135, such as a salt of a tetraalkylammonium halide or a trialkylamine and an organic acid can also be suitably used for the solution to facilitate the film formation, improve the absorption of the amine solution in the support, and accelerate the condensation reaction.
Although the membranes described above possess a relatively high degree of water permeability, it is nonetheless still desirable to further improve the flux of these types of membranes so that they can be operated at lower pressures, such as at 120 psi, in order to conserve energy while still maintaining a high degree of salt rejection.
Other patents of interest include U.S. Pat. No. 4,950,404, inventor Chau, issued Aug. 21, 1990; U.S. Pat. No. 4,761,234, inventors Uemura et al., which issued Aug. 2, 1988; U.S. Pat. No. 4,769,148, inventors Fibiger et al., which issued Sep. 6, 1988; U.S. Pat. No. 5,693,227, inventor Costa, which issued Dec. 2, 1997; U.S. Pat. No. 5,746,917, inventor Altmeier, which issued May 5, 1998; U.S. Pat. No.4,830,885, inventors Tran et al., which issued May 16, 1989; and U.S. Pat. No. 4,980,067, inventors Hou et al., which issued Dec. 25, 1990, all of which are incorporated herein by reference.
With respect to the aforementioned Costa patent, the present inventors note that the N,N-dimethylaminopyridine disclosed therein is said to be used to catalyze an interfacial polymerization reaction between a polyfunctional diamine and a polyacyl halide on a microporous support and is not taught or suggested therein to be used as an acid acceptor or pore former.
A commonly-assigned, co-pending application of interest to the present invention is U.S. patent application Ser. No.09/067,891, filed Apr. 28, 1998, inventors Ja-young Koo and Young Seo Yoon, which application is incorporated herein by reference. The aforementioned patent application relates to, in one aspect, a polyamide membrane that is the reaction product of (i) a polyfunctional amine and (ii) an amine-reactive reactant selected from the group consisting of a polyfunctional acyl halide, a polyfunctional sulfonyl halide and a polyfunctional isocyanate (iii) in the presence of a salt-containing compound, said salt-containing compound being a reaction product of a strong acid and a polyfunctional tertiary amine. Said salt-containing compound is said preferably to be the reaction product of a strong acid and a polyfunctional tertiary amine, wherein said polyfunctional tertiary amine has n tertiary amine groups, n being greater than or equal to 2, and wherein said polyfunctional tertiary amine and said strong acid are reacted together in a molar ratio that is greater than or equal to 1:1, respectively, and is less than 1:n, respectively. A composite reverse osmosis membrane that includes the aforementioned polyamide membrane is made, in accordance with the teachings of said patent application, by (a) coating a porous support with an aqueous solution comprising said polyfunctional amine and said salt-containing compound so as to form a liquid layer on said porous support; (b) contacting said liquid layer with an organic solvent solution comprising said amine-reactive reactant so as to interfacially condense said amine-reactive reactant with said polyfunctional amine, thereby forming a cross-linked, interfacial polyamide layer on said porous support; and (c) drying the product of step (b) at a temperature of 50.degree. C. to 130.degree. C. to form a composite reverse osmosis membrane.
Another commonly-assigned, co-pending application of interest is U.S. patent application Ser. No. 09/172,594, filed Oct. 14, 1998, inventors Ja-young Koo, Young Seo Yoon, Jong-Eun Kim and Nowon Kim, which application is incorporated herein by reference. The aforementioned patent application relates to, in one aspect, a polyamide membrane that is the reaction product of (i) an aqueous solution comprising a polyfunctional amine, a salt-containing compound and one or more polar solvents, said salt-containing compound comprising at least one tertiary amine salt functional group and at least one tertiary amine functional group; and (ii) an organic solvent solution comprising an amine-reactive reactant selected from the group consisting of a polyfunctional acyl halide, a polyfunctional sulfonyl halide and a polyfunctional isocyanate. Said salt-containing compound is said preferably to be the reaction product of a strong acid and a polyfunctional tertiary amine, wherein said polyfunctional tertiary amine has n tertiary amine groups, n being greater than or equal to 2, and wherein said polyfunctional tertiary amine and said strong acid are reacted together in a molar ratio that is greater than or equal to 1:1, respectively, and is less than 1:n, respectively. A composite reverse osmosis membrane that includes the aforementioned polyamide membrane is made, in accordance with the teachings of said patent application, by (a) coating a porous support with said aqueous solution so as to form a liquid layer on said porous support; (b) contacting said liquid layer with said organic solvent solution so as to interfacially condense said amine-reactive reactant with said polyfunctional amine, thereby forming a cross-linked, interfacial polyamide layer on said porous support; and (c) drying the product of step (b) at a temperature of 50.degree. C. to 130.degree. C. to form a composite reverse osmosis membrane.
Although each of the two foregoing commonly-assigned, co-pending applications teaches that the aqueous solution includes, in addition to the polyamine, the reaction product of a strong acid and a polyfunctional tertiary amine, neither of said applications teaches or suggests reacting together said polyfunctional tertiary amine and said strong acid in a molar ratio that is greater than or equal to 1:0, respectively, and is less than 1:1, respectively. In addition, neither of said two applications teaches or suggests drying the interfacial reaction product at a temperature less than 50.degree. C.